Smartphone and tablet computers support powerful, versatile computing and communication. These portable electronic devices can run thousands of different software applications (“apps”), which are a great convenience to users. A person can easily carry such devices and use them whenever and wherever the need arises. However, one important limitation of such devices lies in the display technology. In order to provide readability in low to moderate light conditions smartphones and tablets use emissive displays, such as Liquid Crystal Displays (LCDs) or Active Matrix Organic Light Emitting Diode (AMOLED) displays.
However given the finite efficiency of such displays in converting electrical energy to visible light and given the necessarily limited electrical energy storage (e.g., battery capacity) of portable devices, as a practical matter there is an imposed limit on the brightness and usage per charge of LCD and AMOLED displays. The brightness limits become problematic when using the devices outdoors on bright sunny days. Under such ambient lighting conditions, the unwanted inherent reflectivity of the display in combination with the high ambient light illuminance on the display surface may lead to the displayed image or text being ‘washed out’ and difficult to discern.
In the past, transflective displays have been used on a limited basis. Like other LCD displays, a transflective display includes a 2-D array of pixels and each pixel includes multiple subpixels, for example red, blue and green subpixels. In a transflective display, each subpixel is divided into two parts such that it includes a reflective part and a transmissive part. As in other LCD displays, electric voltages are used to alter the configuration (e.g., the molecular long axis orientation) of the liquid crystals in the display to modulate the passage of light through the display. In the reflective portion, light traverses the liquid crystal twice—once going in and once going out after reflection. On the other hand in the transmissive portion, light from a back light located behind the display traverses the liquid crystal only once on the way out of the display.
In order to try to equalize the effect of electric voltage-induced alteration of the liquid crystal on the light in transmissive and reflective portions, one inside surface of the transflective display is corrugated so that the depth of liquid crystal material in the reflective portion will be half of the depth of liquid crystal material in the transmissive portion thereby equalizing the optical path length through the liquid crystal for transmitted light and reflected light. The step-change in depth creates a distorted region of liquid crystal which lowers display contrast. Additionally, there remains a difference in the voltage to brightness (input-output) functions for the two portions of each transflective subpixel which limits display fidelity.
Recently there has been a trend toward very high resolution displays. Pixel densities greater than 300 pixels per inch (ppi) are not uncommon and significantly higher pixel densities are on the horizon. At such high densities, the distorted region that reduces contrast would have an increased relative size, thereby leading to further reduced contrast. Therefore a solution that provides the benefits of transflective displays and is adaptable for high pixel densities is desirable.
Additionally, as people become increasingly reliant on their smart phones and tablets, they tend to depend on their devices for keeping current on social network updates, calendar events, text messages, email messages, and voice mail messages, for example. It would be desirable to have the devices function so that a person could glance at the screen of their device without having to actuate the display screen and be able to see notifications. However running a light emissive display such as an LCD or AMOLED constantly would drain the battery quickly.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of the embodiments described herein.